- MAXIMUM PERMISSIBLE DEPLETION LEVEL
It is generally considered that for most crops, when the Available Moisture in the root zone has been depleted to 50% of Field Capacity, irrigation should commence in order to avoid crop stress.
Exceptions to the rule do occur. For example in cotton, a 70% depletion level is acceptable for most of the season. This is because the objective is to stimulate flowering and not to produce maximum vegetative growth.
Some farmers ‘harden‐off’ plants by applying water stress at an early stage ‐ in order to encourage downward growth of roots.
This process is known as Reduced Deficit Irrigation (RDI).
Vegetable crops are normally not stressed beyond the 40% depletion level.
- TABULATION OF DATA
The collected data is tabulated into an easily readable chart, so that at any stage and for each crop or land it can be seen at a glance the maximum permissible Eo Deficit (from the pan) before irrigation is necessary.
The Irrigation Scheduling Table for Highveld maize is an example of a crop tabulated for the coming season for growing conditions.
Table 1: Irrigation scheduling table for Highveld maize
| WEEKS AFTER PLANTING | DATE | EFFECTIVE ROOT DEPTH (MM) | AVAILABLE MOISTURE IN ROOT ZONE (MM) (CLAY SOIL) =13,5c4) | Et/Eo RATIO | PERMISSIBLE Eo DEFICIT (MM) (AT 50%) LEVEL | IRRIGATION REQUIRED TO FILL ROOT ZONE |
| INSERT BELOW | FROM GRAPH | C x 13,5/100 | FROM GRAPH | Dx50 / FL x100 | D x 50 / 1 0 0 | |
| A | B | C | D | E | F | G |
| 1 | ( N o ) | 13 | IRRIGATION DURING FIRST FEW WEEKS BY FEEL AND EXPERIENCE | |||
| 2 | (200) | 27 | ||||
| 3 | (300) | 40 | ||||
| 4 | (350) | 47 | 0.30 | 78 | 23 | |
| 5 | 400 | 54 | 0.30 | 90 | 27 | |
| 6 | 450 | 60 | 0.50 | 60 | 30 | |
| 7 | 460 | 64 | 0.70 | 45 | 32 | |
| 8 | 500 | 67 | 0.90 | 37 | 33 | |
| 9 | 550 | 74 | 1.00 | 37 | 37 | |
| 10 | 600 | 81 | 1.10 | 36 | 40 | |
| 11 | 650 | 87 | 1.10 | 39 | 43 | |
| 12 | 650 | 87 | 1.00 | 43 | 43 | |
| 13 | 650 | 87 | 1.00 | 43 | 43 | |
| 14 | 650 | 87 | 1.60 | 43 | 43 | |
| 15 | 650 | 87 | 0.95 | 45 | 43 | |
| 16 | 650 | 87 | 0.95 | 45 | 43 | |
| 17 | 650 | 87 | 0.90 | 48 | 43 | |
| 18 | 650 | 87 | 0.90 | 48 | 43 | |
| 19 | 650 | 87 | 0.80 | 54 | 43 | |
| 20 | 650 | 87 | 0.80 | 54 | 43 | |
| 21 | 650 | 87 | 0.80 | 54 | 43 |
Example:
- From the table on 16th February, the crop is in its 17th week of growth. The theoretical root depth is 650mm. As the Available Moisture of the soil is 13.5% there is
13.5% (Available moisture) x 650mm (Effective roofing depth) /100 = 87mm of water available to the crop if soil is at Field Capacity. (D)
- Accepting a 50% Depletion Level as the desirable maximum, then irrigation should commence when D x 50/100 = 87 x 50/100 = 44mm of soil water has been lost by the crop. (G)
However at this stage Et/Eo = 0.9. (FL)
- Thus, permissible Eo (from pan) D x 50 /E x 100= 87 x 50/0.9 x 100 = 4350/ 90 = 48mm (F)
- So, when there has been a total Pan loss (since last irrigation) of 48mm then 43 mm of water should be applied.
- SCHEDULING IN PRACTICE
A calculation such as the one above could be done for any stage of the crop. In practice, it has been found that determination of Permissible Eo Deficits on a weekly basis is desirable, as indicated in the scheduling table.
If a scheduling table is drawn up for each crop at the start of the season, then the actual calculation is eliminated during the season. All one needs to do is check the Permissible Eo value against the current value for cumulative Eo since last irrigation.
The use of a Control Indicator Board further simplifies the process.
As watering during the first 3 weeks is necessary to ensure proper germination, irrigation is then up to the irrigator’s own judgement and experience. The use of the scheduling board during the first 3 ‐ 4 weeks will not give satisfactory results.
Daily recording of Eo and rainfall are necessary but the scheduling board need only be visited once every few days.
RAINFALL
Daily rainfall figures should be subtracted from the cumulative Eo figure. Never consider falls of less than 10mm per day.
Although it is not strictly correct to do so, in practice, a rainfall of for example, 20mm will be subtracted from the current Eo total since the last irrigation, regardless of what the crop Et/Eo ratio is at the time.
Remember, when subtracting rainfall from the cumulative Eo figure, not to subtract more rainfall than could be used to fill the crop root zone to maximum depth (even if the rainfall is in excess of that amount). For example, if a mature maize crop has lost only 10mm of water since the last irrigation and a rainfall of 20mm occurs, only 10mm should be subtracted from the evaporation pan total.
Also to be remembered is that some evaporation will usually take place on the same day as the rain falls.
This can best be estimated and added to the cumulative Eo figure (as the Eo pan seems not to show a true reading of rainfall). Undoubtedly, scheduling is much easier in the dry season or the low rainfall areas.
4. THE CONTROL INDICATOR BOARD
This visual aid to scheduling is invaluable as it shows at a glance a picture of the present irrigation priorities for a number of blocks of land, even several days in advance.
There are two main types of indicator board:
- The Peg Board; and
- The Cumulative Evaporation Board
Reference can be found to the Peg Board type in several publications. The reasons for the use of Type (b) are as follows:
- It is found easier to operate than (a). Only one item has to be moved each time the board is visited;
- It is easier to read and gives a better indication of priorities for water than (a);
- The paper strips for each crop serve as a record of irrigation frequency and quantity for use in future years; and
- With a large variety of crops grown it is essential to have a constant ‘picture’ of priorities for water use.
A description of the Cumulative Evaporation Board system follows:
The daily evaporation is recorded on a separate chart and a running (cumulative) total is kept on that same chart. Rainfall is subtracted from this total. At any time one can visit the Evaporation Chart (Table 2) and read off the cumulative Eo value to that date.
Table 2: Evaporation Record Chart
| Date | Daily Eo | Daily Rainfall | Cumulative Eo |
| 1st October 2012 | 6.0 | 6.0 | |
| 2nd | 7.2 | 13.2 | |
| 3rd | 7.0 | 20.2 | |
| 4th | 5.9 | 26.1 | |
| 5th | 7.1 | 33.2 | |
| 6th | (2.0) | 25.2 | 8.0 |
| 7th | 6.0 | 14.0 | |
| 8th | 6.2 | 20.2 | |
| 9th | 5.9 | 26.1 |
On the Control Indicator Board (Figure 1 below); each land is represented by a vertical column. In this example only three different lands are shown. At each side of the board is a further vertical column of numbers representing cumulative evaporation since 1st October that season.
A horizontal string (marked in red) is moved up the board to indicate the present value of Cumulative Eo (obtained from Table2). It is held by “pins” in softboard columns at either side.
Irrigation Scheduling Tables for each crop are drawn up early in the season. From these tables it is a simple matter to mark, for each crop column, the lines to indicate the point at which Maximum Permissible Eo will have taken place. E.g. if M.P. Eo = 45mm, then a horizontal line is drawn across that column, exactly 45mm above the line indicating the last irrigation.
When the string strikes one of these lines, it is time to irrigate that crop again.
In the example of Figure 1, the string has reached 425mm Cumulative Eo. It has struck a horizontal line on Land 3, so the beans must be irrigated. Next to be irrigated is the maize on Land 1, in approximately 3 days time (at 4mm of Eo per day). The lucerne still has 50mm Eo to go before irrigation is necessary. This would give time for forage cut on this land.
Figure 1: An example of a Control Indicator Board
It is convenient to let the horizontal lines in the columns represent the first day of the irrigation cycle on that crop.
N.B. This system breaks down during periods of much rainfall.
